1. Technical Field
The present disclosure relates to a common mode noise reduction circuit and, more particularly, to a common mode noise reduction circuit for suppressing common mode noise while maintaining the differential mode signals with minimum loss.
2. Description of Related Art
The advance of technology has driven faster operation speed and clock frequency for digital circuits such that the techniques of differential microstrips and striplines are widely applied in high-speed data transmission applications. Ideally, a differential transmission line has the characteristics of low electromagnetic radiation, and low crosstalk. However, in an actual circuit, unbalanced (or asymmetric) structure may inevitably carry unwanted common mode noise attached to the differential signals.
In general, it is needed to design a circuit with asymmetrical wiring in order to reduce the layout area. Any by-pass, feed-through, or slots on the circuit board may cause discontinuity problem, and generate unbalance in terms of amplitudes and phases when outputting signals. The problem is that the asymmetrical circuit structure may adversely convert differential mode signals into common mode noise, where the common mode noise is carried, through the ground contact, to the border of the circuit, the connected wires, or the metal layer of shielding, and worse yet, the common mode noise may cause serious electromagnetic interference, an impact to the radio frequency of the circuit or the antenna operation.
Referring to FIG. 1, a circuit diagram of a common mode noise suppression circuit 10 in the art is shown. The circuit 10 includes a common mode choke 11, a first input end 131, a first output end 133, a second input end 151, and a second output end 153. The circuit 10 suppresses common mode noise by generating high inductive impedance for the common mode noise via the common mode choke, which is made of magnetic material. However, the permeability of the magnetic common mode choke 11 attenuates quickly in high frequency section, a limitation that makes the common mode choke 11 not suitably applied in high-speed transmission interface with frequency section of GHz or above.